Virologic and immunologic factors in the pathogenesis of subacute and chronic measles encephalitis will be investigated. Strains of measles virus and cell cultures containing defective measles virus genome have been isolated in our laboratories from brains of patients with subacute sclerosing panencephalitis (SSPE). These viruses and cells will be characterized using a combined biochemical-immunologic-genetic approach. Mechanisms involved in depression of cell mediated immunity by measles virus will be studied. Using a variety of techniques, attempts will be made to detect virus or viral products in brains of patients with subacute and chronic nervous system diseases, other than SSPE and multiple sclerosis.